Protozoa are single-celled organism or eukaryotes (which are organisms whose cells contain membrane-bound organelles and nuclei). Other eukaryotes include us, other animals, and plants. Eukaryotes also include other microorganisms: algae, helminths, and fungi. Protozoa are found everywhere. They can survive their own as free-living organisms within the environment, often within the soil, water, or moss. They can even be resting cysts, which lets them survive through dry times. Some are parasites. Others live in symbiosis with other organisms; each relies on the other for survival. Protozoans cause some common infectious diseases like malaria, giardia and toxoplasmosis. These infections are found in very different parts of the body — malaria infections start in the blood, giardia starts in the gut, and toxoplasmosis can be found in lymphatic nodes, eyes, and also our brain. Likewise, sleepy sickness is caused by a protozoan infection, as is dysentery thanks to Entamoeba histolytica. Infections caused by protozoa are often spread through ingestion of cysts, sexual transmission, or through insect vectors. There are many common—and not so common—infections caused by protozoa. Some of these infections cause illness in many people each year; other infections are rare and hopefully disappearing.
